1. Field of the Invention
The present invention relates to a braking device employed for a two-wheeler such as a bicycle or a motorcycle, and more particularly, it relates to a roller brake or drum brake for braking a two-wheeler under dry frictional sliding conditions without a solid or semisolid lubricating component such as grease or oil.
2. Description of the Background Art
FIGS. 1A and 1B show a conventional drum brake for a two-wheeled vehicle, which is generally called a roller brake for a two-wheeler herein. This roller brake comprises a guide case 1 which is fixed to a wheel and rotates with this wheel, and a brake shoe 2 which is pressed against the inner peripheral surface of the guide case 1 to exhibit a braking effect. The brake shoe 2 has an outer peripheral surface which is substantially identical in radius of curvature to the inner peripheral surface of the guide case 1.
In the conventional roller brake, both the guide case 1 and the brake shoe 2 are made of steel or an iron alloy. In order to prevent a discomfort phenomenon such as squeaking or chattering during braking, seizing of the guide case 1 and the brake shoe 2, or locking causing adhesion between the guide case 1 and the brake shoe 2, the inner peripheral surface of the guide case 1 is grooved in the conventional roller brake for allowing intervention of a solid or semisolid lubricant such as grease or oil.
When the lubricant such as grease intervenes between the guide case 1 and the brake shoe 2, however, the braking force is disadvantageously reduced although the lubricity is improved. In this case, the roller brake can exhibit only a small braking force of about 0.1 in terms of the friction coefficient, for example. If maintenance such as supplementation of the lubricant such as grease is neglected, the grease or the like intervening between the guide case 1 and the brake shoe 2 is used up, which will result in locking of the brake shoe and the guide case.
Recent high-performance two-wheelers aim to avoid the use of the lubricant such as grease, in order to improve the braking force for attaining a friction coefficient of at least about 0.1. However, it has been recognized that locking is caused by adhesion when a two-wheeler is braked under dry sliding conditions without intervention of grease in case of employing a conventional brake material such as a friction material for a brake disclosed in Japanese Patent Laying-Open No. 56-133441 (1981), 56-120787 (1981), 2-11936 (1990) or 5-331451 (1993), for example. Thus, further improvement is necessary for a brake material to be used in such an application.